WASHINGTON (CNN) -- With 140,000 American troops now deployed and thousands more awaiting orders, the war in Iraq is a major theme in this year's presidential campaign.

It's the first time since the 1972 election, when the United States was still involved in the Vietnam War, that candidates are stumping for votes as significant numbers of service members are in harm's way. (Special Report: The issues)

In May 2003, President Bush stood aboard the aircraft carrier USS Abraham Lincoln and declared an end to "major combat operations" under a banner that read "Mission Accomplished." Since then, the war in Iraq has been a major focus of his re-election campaign.

Bush's Democratic opponent isn't shying away from the military either as he campaigns. Sen. John Kerry of Massachusetts officially declared his candidacy while standing in front of a decommissioned aircraft carrier in South Carolina last September.

Both candidates mention the war in Iraq, terrorism, veterans and military service frequently in their campaign speeches. And the issues come up in ads as well.

So far, 27 of 41 ads aired by the Bush-Cheney campaign have touched on military issues. Kerry has mentioned such issues in 20 of his ads since the start of the year.

Defense is an issue that, like many others in this campaign, has the American voting public split down the middle.

A CNN/USA Today/Gallup Poll in June asked which presidential candidate would better handle "the situation in Iraq?" Forty-seven percent of the respondents picked President Bush and 46 percent picked Kerry, a statistical tie. A poll released Thursday on the same question had Bush at 49 percent and Kerry at 44 percent.

On the eve of the Democratic National Convention, the election remains too close to call. This week's USA Today/CNN/Gallup Poll shows Kerry holding a 49 percent to 47 percent edge among likely voters. With independent candidate Ralph Nader in the picture, Kerry's margin over Bush drops to a single point.

Bush and Kerry are tied on other basic political measures as well, the poll shows.

Their favorable ratings are within three percentage points of each other, and a roughly equal number of respondents said they believe the two men have the right personality and leadership qualities to be president. Roughly equal numbers also said that both men agree with their views on major issues.

Differences on defense

But the poll shows some differences when it comes to America's defense. Kerry is seen as better on the economy and health care; Bush is seen as better able to handle terrorism and Iraq. However, more than six in 10 respondents said they would have confidence in either Kerry or Bush to protect the nation from terrorism; a third said Kerry has a clear plan for Iraq.

Bush is seen as a stronger leader, more knowledgeable about the issues, and less likely to flip-flop, the poll shows. Kerry is seen as caring more about the average American and more respected by world leaders.

In previous years, conventions have boosted the challenger on a number of these issues, and the schedule for the conclave in Boston indicates the Democrats will try to move Kerry's numbers on many of these factors.

Kerry has tapped fellow Vietnam veteran and former U.S. Sen. Max Cleland of Georgia to introduce him during the crowning July 29 session of the convention.

Cleland's introduction will highlight the theme of the convention: "Stronger at Home, Respected in the World." Kerry's swift boat crew mates from his time in Vietnam also will appear during the convention.

"Obviously, we see the convention as a real opportunity to raise John Kerry's profile on the national stage," a Kerry campaign aide said.

Campaigning limits

While the military may be a key issue in the election, campaigning on military bases is forbidden, according to Pentagon spokesman Glenn Flood.

However, since both candidates and their running mates are elected officials, they may attend events on military bases, but they must refrain from soliciting support or votes while on base. It is a "gray area," Flood said.

The men most identified with the war in Iraq and the U.S. military are trying to stay above the fray.

Publicly, Defense Secretary Donald Rumsfeld has said he will adhere to the tradition that the Pentagon and defense chief do not become involved in politics.

A senior official said Rumsfeld in recent meetings has turned to Gen. Richard Myers, chairman of the Joint Chiefs of Staff, and said, "You shouldn't be involved in this" as conversations take political tones.

"The president, when I took the job, asked me and Secretary [of State Colin] Powell to stay out of politics," Rumsfeld told reporters this week, adding that neither he nor Powell is planning to attend the upcoming Republican National Convention in New York.

"We're in a war. We've got serious business to be doing," Rumsfeld said. "And it's perfectly understandable that our country goes out and has a big debate and discussion on the election and that's a good thing. It's how it works in our country.

"And by the same token, we've got to keep our eye on the ball, and the ball is trying to protect the American people from additional terrorist attacks and try to track down the folks that are planning threats against our country, here and abroad."